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I am seeing the same behaviour. It is 100% reproducible with one debian guest system and not reproducible with another quite similar debian guest system.
The samll window can be rezised. After resizing I see about two pixels of the title bar. So I can grab and move it. It does not show any content. If I click on the close symbol on the top right side, I can choose to exit.

Clicking in the (empty) window at the top left corner - where the menu should be shown - it's possible to switch to "full screen mode" (the menu is hidden, but accessible). Then the content/ the guest screen is restored. After switching back from "full screen mode" everything seems to work again.

This has happened to me on 2 separate Windows 7 x64 hosts, and happens very frequently with a single guest. This guest was migrated from one system to another (using backed up XML) and seems to reproduce the problem frequently on the new machine.

It doesn't always happen - probably 75% of the time. It will usually restore, but no window appears. I must right click -> show on the guest in the main Virtualbox window. It starts at a very small size and must be resized.